Solid oxide cells (SOC's) generally include cells designed for different applications, such as solid oxide fuel cells (SOFC's), solid oxide electrolysis cells (SOEC's), or membranes. Due to their common basic structure, the same cell may, for example, be used in SOFC applications as well as SOEC applications. Since in SOFC's fuel is fed into the cell and converted into power, while in SOEC's power is applied to produce fuel, these cells are referred to as ‘reversible’.
Solid oxide fuel cells (SOFC's) are well known in the art and come in various designs. Typical configurations include an electrolyte layer being sandwiched between two electrodes. During operation, usually at temperatures of about 500° C. to about 1100° C., one electrode is in contact with oxygen or air, while the other electrode is in contact with a fuel gas.
The most common manufacture processes suggested in the prior art comprise the manufacture of single cells. Generally, a support is provided, on which an anode layer is formed, followed by the application of an electrolyte layer. The so formed half cell is dried and afterwards sintered, in some cases in a reducing atmosphere. Finally, a cathode layer is formed thereon so as to obtain a complete cell. Alternatively, one of the electrode layers or the electrolyte layer may be used as a support layers, having a thickness of about 300 μm or more.
This approach requires a relatively thick support layer to provide mechanical stability of the obtained cell, thereby increasing the overall thickness of the single cells. Further, to obtain high voltage and power, many cells are stacked together in series. However, a large thickness of the individual cells will limit the cell performance and will decrease the power/volume or power/weight of the cell stack. Furthermore, a large thickness also translates into use of more material and thus adds to the overall costs of the stack.
US-A-2004/00115503 discloses an electrochemical device assembly, comprising a porous electrically conductive support layer; a prefabricated electrochemical device layer; and a bonding layer between said support layer and said electrochemical device layer. The conductive support layer has a thickness of from 50 to 750 μm.
US-A-2002/0048699 relates to a SOFC, comprising a ferritic stainless steel support substrate including a porous region and a non-porous region bounding the porous region; a ferritic stainless steel bi-polar plate located under one surface of the porous region of the substrate and being sealingly attached to the non-porous region of the substrate about the porous region thereof; a first electrode layer located over the other surface of the porous region of the substrate; an electrolyte layer located over the first electrode layer; and a second electrode layer located over the electrolyte layer. The substrate preferably has a thickness of from 50 to 250 μm.
US-A-2004/0166380 relates to a method of producing porous electrodes for use in solid oxide fuel cells. The electrodes are formed from a powder of the electrolyte material, and tape cast to form a two-layer green tape. One of said layers will be the later electrode layer, the other layer the electrolyte layer. The obtained green tape is then sintered to form a porous matrix of the electrolyte material near the surface of the first layer and a dense layer of the electrolyte material from the second layer. The final electrode is formed by impregnating the porous portion with electron conducting material.
U.S. Pat. No. 5,273,837 relates to thermal-shock-resistant fuel cell designs comprising flat and corrugated ceramic sheets combined to form channelled structures, the sheets being provided as thin, flexible ceramics. Said flexible, pre-sintered ceramic sheets are used as electrolytes or electrolyte substructures and can be produced as free-standing sheets of high strength but very slight thickness not exceeding about 45 μm. Combined with the electrode layers, the thickness of said substructure does not exceed 150 μm.
However, in view of the increasing importance of solid oxide fuel cells as alternative energy converters, there is a desire for SOFC's with improved performances as compared to the cells provided by the prior art so far.
Similar to the above described solid oxide fuel cell designs, separation membranes comprise a thin membrane layer sandwiched by electrodes.
Such separation membranes may, for example, be used to produce synthesis gas, which is a mixture of CO and H2. Air and methane are supplied at the cathode and anode, respectively, and synthesis gas is obtained via a partial oxidation of the methane. Separation membranes may also used for hydrogen separation for the production of high purity hydrogen. In this case the membrane material must be proton conducting.
Usually, a support layer having a thickness of about 300 μm or more is used to support the membrane and to provide the required strength. Alternatively, one of the electrode layers may be used as the support, being of corresponding thickness. For example, metal electrodes have been proposed as an electrode material since metal is mechanically more robust than a ceramic layer.
However, there is a desire for thin, and in principle unsupported ceramic separation membranes with improved performances as compared to the membranes provided by the prior art so far.
WO-A-2006/082057 relates to a method of producing a reversible solid oxide fuel cell, comprising the steps of 1) providing a metallic support layer; 2) forming a cathode precursor layer on the metallic support layer; 3) forming an electrolyte layer on the cathode precursor layer; 4) sintering the obtained multilayer structure; 5) impregnating the cathode precursor layer so as to form a cathode layer; and 6) forming an anode layer on top of the electrolyte layer.
WO-A-2005/122300 relates to a SOFC cell comprising a metallic support ending in a substantially pure electron conducting oxide, an active anode layer consisting of dopedceria, ScYSZ, Ni—Fe alloy, an electrolyte layer consisting of co-doped zirconia based on an oxygen ionic conductor, an active cathode layer and a layer of a mixture of LSM and a ferrite as a transition layer to a cathode current collector of single phase LSM.
US-A-2006/025718 discloses a fuel cell electrode material comprising a cermet which comprises metal particles consisting of cobalt and nickel and electrolyte particles consisting of solid oxides, wherein said metal particles comprise 20 to 90 mol % cobalt and the residue of nickel in terms of CoO and NiO, respectively.
U.S. Pat. No. 6,017,647 discloses a composite oxygen electrode/electrolyte structure for a solid state electrochemical device having a porous composite electrode in contact with a dense electrolyte membrane.
GB-A-1000576 relates to a gas electrode for fuel cells which comprises a body of porous sintered electrode material presenting a pair of oppositely disposed surfaces, the electrode material being electrochemically active, a network of gas channels centrally disposed in said body between said surfaces, and a plurality of bridges of porous sintered material integral with and connecting said active surfaces throughout said central gas channels, the gas channels having a cross-sectional area substantially greater than that of the pores of the porous sintered material, and being substantially evenly distributed throughout the electrode between active surfaces.
U.S. Pat. No. 5,273,837 discloses a thermal-shock-resistant fuel cell design comprising flat and corrugated ceramic sheets combined to form channelled structures, the sheets being provided as thin, flexible ceramics and being particularly effective when used as components of compliant electrolyte substructures incorporating the flexible ceramics with fuel cell electrodes and/or current conductors bonded thereto.